staging:iio:light:tsl2x7x drop assignment of driver_module

The equivalent is now done via macro magic in the register call.

Note this is the only case not found by the coccinelle script
suggesting that perhaps that script needs to be a little more
clever!

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>